A variety of golf balls designed to provide a wide range of playing characteristics, i.e., the compression, velocity, “feel,” and spin, that can be optimized for various playing ability, are known in the prior art. Conventional golf balls include solid golf balls having one or more layers. Configurations of solid golf balls include one-piece balls, two-piece balls and multilayer golf balls. One-piece balls are generally easy to manufacture and relatively inexpensive. However, one-piece balls have poor playing characteristics and are typically limited for use as range balls. Two-piece balls are constructed with a solid core and a cover surrounding the core. The durability and driving distance associated with two-piece balls makes them very popular with recreational golfers. Balls having a two-piece construction are commonly formed of a polymeric core encased by a cover. Although two-piece balls are generally easy to manufacture and popular with recreational golfers, two-piece balls have limited playing characteristics. Multilayer golf balls contain a solid core having one or more layers and a cover surrounding the core. The cover may also contain one or more layers. Multilayer golf balls provide an extended range of playing characteristics over two-layer balls.
In addition to the configuration of the core and cover of solid golf balls, the materials that make up the core and cover also affect the playing characteristics of solid golf balls. One of the most common polymer components used in golf ball construction is polybutadiene, in particular polybutadiene having a high cis-isomer concentration. The use of a polybutadiene having a high cis concentration in a golf ball core results in a very resilient and rigid golf ball, especially when coupled with a hard cover material. These highly resilient golf balls have a relatively hard “feel” when struck by a club. Often, a softer feel is desired; however, soft feel golf balls constructed with a high cis concentration polybutadiene have low resilience. Since, resilience is sacrificed for ball feel, other formulations of polybutadiene have been attempted to provide golf balls having both improved resilience and a soft feel.
Polybutadiene, or butadiene rubber, having a high trans content, for example from about 20% to about 60%, and a low vinyl content, for example less than about 5% or preferably less than about 2%, exhibits superior resilience at a given compression. In addition, this polybutadiene has a high molecular weight, for example greater than about 350,000 and preferably greater than about 500,000, and a low polydispersity, for example a molecular weight distribution less than about 3 preferably less than about 2.
The desired trans content or isomer ratio in the polybutadiene can be achieved by cis-to-trans conversion. For example, the polybutadiene can be converted from cis-to-trans when the polybutadiene is cured. Cis-to-trans conversion during cure, however, results in a trans gradient that roughly mirrors the cure gradient, because the two chemical reactions occur simultaneously, making it difficult to alter or reverse the trans gradient. Alternatively, polybutadiene can be directly polymerized to the desired isomer ratio. The direct polymerization method, however, can result in reduced molecular weight and high polydispersity, among others. In another method, a polybutadiene having a high trans content, greater than about 90%, is solution blended with a polybutadiene having high cis content, greater than about 90%. However, due to the incompatibility of these two polymers, crystallinity, which is believed to be bad for golf ball properties, is not completely eliminated.
For example, polymers that have a high percentage of the trans-polybutadiene conformation, such as DIENE 35NF, from Firestone Corp., that has 40 percent cis-isomer and 50 percent trans-polybutadiene isomer, and mixtures of high-cis and high-trans-polybutadiene isomers, such as CARIFLEX BR1220, from Shell Corporation, and FUREN 88, from Asahi Chemical Co., respectively, typically do not yield high resilience values and therefore are not desirable.
It is thus desired to prepare golf balls having lower compression, i.e., a softer ball, while having the same or higher resilience than conventional balls. It is alternatively desired to obtain the same or lower compression while achieving greater resilience. To this end, it is believed that solution converted, post-polymerization of polybutadiene will avoid the above-mentioned problems exhibited by conventional trans polybutadiene processing and will yield superior golf ball materials.